The abrading effect is produced almost entirely by the simple physical process of the harder substance shearing or fracturing small chips off the work-piece to smooth it.
Abrasives are used in three main ways. One is to use the abrasive material directly on a substance: sharpening a knife on a grinding wheel is an example of this. Another is to coat another substance, such as a piece of paper, cloth or rubber on a metal disc, with granules of abrasive material, and use this as a tool; sandpaper is the commonest application of this technique. The third method is sandblasting or grit blasting, where a powerful stream of air containing abrasive particles is directed at an object to abrade its surface.
Apart from their use in sharpening-stones and grinding wheels, direct-action abrasives are also used in powder form. Most domestic cleaning agents (except soap and washing powder) contain abrasives, which are generally silica, pumice or aluminum oxide ground to a very fine powder. The chemical action of the cleaning agent is helped by the abrasive, and the two substances clean faster than either would alone.
Toothpaste also contains a mild abrasive, which is generally finely powdered chalk. Old-fashioned tooth powder often contained powdered pumice or silica, which wore the enamel off the teeth in a short time, but manufacturers now claim that the cleaning action of their product is mostly chemical.
Most abrasives used in industry are applied indirectly by being stuck to a backing. This saves expense because less abrasive is used.
The simplest type of coated abrasive material is sandpaper, which is made by simply gluing granules of abrasive material to a sheet of paper.
Abrasive papers are made in a vast range of types, and have many uses. One unusual application is in the printing industry, where a sheet of paper to be printed on both sides is laid on a sheet of abrasive paper called tympan (abrasive side up) while the second side is printed. In this way, the printed side can rest on the abrasive points, which hold it steady and prevent the wet ink from smearing.